Place the hazelnut butter in a small microwave-safe bowl and warm it in the microwave for about 15 seconds, just until it loosens up and becomes easy to stir. Set it aside to cool to room temperature so it does not melt the cream later.
In a large chilled bowl, combine the cold heavy whipping cream, cocoa powder, and powdered erythritol. Beat with an electric mixer on medium speed for about 1 minute until the cocoa is incorporated and the mixture starts to thicken.
Increase the speed to high and continue whipping until soft peaks begin to form. Add the slightly cooled hazelnut butter and vanilla extract, then continue beating until the mousse reaches stiff peaks and has a rich, fluffy texture. Be careful not to over-whip past stiff peaks.
Spoon or pipe the mousse into 4 individual serving cups or glasses. Refrigerate for at least 30 minutes to let the mousse firm up and develop its best flavor.
Just before serving, top each mousse cup with a sprinkle of chopped toasted hazelnuts and a few sugar-free chocolate shavings.
